It’s all about PageRank.
Google sets crawl budget based primarily on your site’s authority. That means PageRank of individual pages. Sites with a higher average PR across their pages will see a larger crawl budget; Google will crawl more URLs on these sites.
My brief synapses
The basic thing to remember is that the higher your page rank is the longer googlebot will stay in your site and crawl and index your pages.
There are a few other nice nuggets of search info in this article too.